This invention pertains to the design and manufacture of large, flat-panel electronic displays and, more particularly, to the manufacture of active matrix liquid crystal display (AMLCD) type, flat-panel displays assembled in a single monolithic or monolithic-like assembly, strengthened for structural integrity, corrected for brightness and hue variations due to optical or electro-optical aberrations and structural non-uniformities, equipped with lighting means and optical means that provide large view angles while improving visual acuity and contrast, and thin film wiring in the display that is uniquely designed to avoid or compensate for non-uniformities in pixel response, brightness, and chromaticity.
Large displays can be constructed using several established display technologies, including cathode ray tubes (CRT) and projectors of the rear view type. However, as the diagonal size of these displays increases their volume and weight increase significantly. Further, manufacturing becomes more difficult while the manufacturing cost greatly increases.
An alternative approach to implementing large direct-view displays is provided by flat-panel displays, which offer a much reduced thickness and weight. The active-matrix liquid-crystal display (AMLCD) is the most mature of these technologies. The structure of an AMLCD consists of a liquid crystal (LC) layer sandwiched between two thin glass plates with a thickness typically of 0.5 mm, 0.7 mm, or 1.1 mm. As the diagonal size of the AMLCD increases above about 20 inches, the structural integrity of the sandwich becomes insufficient; hence the mechanical assembly should be strengthened for larger sizes. At the same time, non-uniformities arising from manufacturing and operation dramatically increase and the manufacturing yield decreases.
Today""s AMLCD displays have several additional drawbacks in consumer applications. In particular, the view angles are limited to values much smaller than those of the CRT. In addition, the brightness-energy efficiency is reduced by polarizers, light collimator means, and any screens used to enhance the view angles. If such direct-view AMLCDs are to compete with CRTs in consumer applications, these deficiencies must be overcome.
Recent improvements in display technologies, as disclosed in the above related patents and patent applications, have been made to overcome these deficiencies or compensate for them in large tiled AMLCDs, in which the display is assembled from several smaller, independently fabricated pieces or tiles. Many of these improvements can also be applied to large monolithic displays. These improvements can help improve the characteristics of a monolithic or monolithic-like display or compensate for artifacts resulting from imperfect manufacturing of the components or their assembly. However the substantial structural differences between monolithic and tiled displays must be considered, when the new techniques are applied to monolithic ones.
Unlike tiled displays, monolithic displays have no structural discontinuities in the seams between adjacent tiles, a fact that substantially relaxes light collimation requirements, one of the key techniques used to hide the seams in tiled displays. As a consequence, the useable aperture ratio increases, screen specifications are altered, and the need for masks decreases. Therefore, the design of the optical stack and lighting in large monolithic or monolithic-like displays is significantly different compared to large, tiled AMLCD displays.
The present invention reflects unique designs and methods for fabricating or operating large monolithic or monolithic-like AMLCDs of both color and gray-scale types using many of the techniques developed for large, tiled, flat-panel displays (FPDs). Although this specification describes most of the techniques and methods in the context of AMLCDs, many of them can be applied to other transparent, light-valve type FPDs. Characteristic of such displays is that light from a uniform, back light source is transmitted through the display assembly towards the viewer located on the front side. The light valves control the amount of primary light rays transmitted through the apertures of sub-pixels. The transmitted light from the sub-pixels mixes to form all desired brightness and hue combinations (color space) before it reaches the viewer located at a predefined viewing distance from the display. The techniques and methods transferred from tiled to large-scale, monolithic, flat-panel displays, augmented with other methods described herein, significantly improve the performance of the latter, including viewing angle, image acuity, contrast, and color uniformity. At the same time, these unique design improvements can be used to increase the manufacturing yield, compensate for imperfections arising from the fabrication and assembly of the display, and transform the fragile, large, monolithic display into a robust laminate between glass cover plates and back plates.
Robust display glass laminates can be made using adhesive films with a preferred thickness in the range from 25 to 250 xcexcm or thicker, and optimized in compliance. The monolithic display panel, for example, can be laminated between glass cover and back plates without stressing the birefringent AMLCD glass or deforming the LC cell gap. U.S. Pat. No. 5,867,236 xe2x80x9cCONSTRUCTION AND SEALING OF TILED FLAT-PANEL DISPLAYSxe2x80x9d, copending U.S. patent application Ser. No. 09/490,776 and U.S. patent application Ser. Nos. 09/368,921 and 09/369,465, show laminate structures that accomplish the desired result. Laminate structures for a prototype 800xc3x97600 SVGA 38.6xe2x80x3 diagonal tiled display are also shown in copending patent application Ser. No. 09/368,921. A method for processing large display laminates is discussed in copending patent application Ser. No. 09/322,047 and U.S. Pat. No. 6,097,455.
The laminate is designed with a symmetry about the image creation plane in the AMLCD glass sandwich (LC layer), which contains the weakest link to shear or bending. This link is formed by a narrow adhesive seal, typically about 5 xcexcm thick, that joins the thin-film-transistor (TFT) substrate to the color-filter (CF) substrate around the perimeter of the display. The width of this seal may be as narrow as 1 mm or less, and it may be the only mechanical link, other than surface tension of the LC liquid layer, that holds the substrates together during handling, assembly, and field use induced stresses.
An external full face seal material, in dry film form, or a layered combination with dry film, or liquid film alone, of a preferred thickness range and preferred elastic compliance, is used to bond the robust glass cover and back plates on both sides of the AMLCD sandwich to increase the bending strength. This preferred design provides a substantially increased resistance to bending, thereby decreasing the effect of any unintentional stresses exerted on the narrow seal. The preferred thickness of the adhesive films between cover and back plates and the AMLCD sandwich are dependent on whether a mask is used on the back plate to set the light collimation angles. The relationships of these angles are shown in detail in the aforementioned patent applications.
Since such aperture masks are not needed on the back plate to direct light rays away from the seams in monolithic displays, as in tiled ones, the thickness requirement for the adhesive may be relaxed for the monolithic display laminates. Aperture masks on the cover and back places may still be desirable in monolithic displays to optimize the visual acuity and contrast of displayed images. Alternatively, they may be removed from the display stack, if other light collimation means are preferred.
The air is controllably purged at the meniscus of the full face adhesive interface between cover and back plates and the AMLCD sandwich so that no bubble-type defects are introduced in these laminate structures during assembly. Techniques for achieving bubble free laminate assemblies for attaching the compliant adhesive films to monolithic displays, have been described in copending patent application Ser. No. 09/322,047 and U.S. Pat. No. 6,097,455.
Back or cover plates may be made of standard glass, such as Corning 1737, that is commonly used in the AMLCD industry. Today""s glass sheet thicknesses have been standardized to 0.5, 0.7 and 1.1 mm. Any of these glass thicknesses may be used in display assembly lamination as cover and back plates. Smaller glass thicknesses allow a smaller radius to be used in an adhesive extrusion process as described in U.S. Pat. No. 6,097,455. This translates into a smaller probability for trapping bubble defects. Lamination between glass cover and back plates produces a robust assembly with a constant refractive index and well matched thermal expansion characteristics compared to the conventional display glass substrates. Although robustness is improved by increasing the cross-sectional inertia to bending stresses, it is also important to maintain the neutral axis in the LC image plane, in which the epoxy seal joins TFT and CF substrates. This can be accomplished by making the thickness of the adhesive films equal between the cover and back plates and the AMLCD sandwich. For this reason, the laminate is designed to be approximately symmetrical around the LC plane. Consequently, the thickness of both the glass cover and the plates is chosen to be approximately 1.1 mm or greater.
The transparent regions in the sub-pixel compared the total area of the pixel (ratio defined as aperture ratio) are substantially larger in monolithic AMLCDs (60-80%) compared with tiled displays (30-50%) due to the space required by the seams and tiling functions as described in patent application Ser. No. 60/177,448.
The viewing angles in monolithic AMLCDs are generally not as wide as desired. The image acuity is highly dependent on the back light, which is normally diffuse. One published method under development for improving the viewing angles is discussed in Information Display Magazine, February 1999 by Joel Pollock, xe2x80x9cSharp Microelectronics Approach to New-Generation AMLCDsxe2x80x9d. Even though in-plane switching, as discussed in this reference, has drawbacks, including slower response times, this technology is gaining favor because there is no other fully developed method to meet the wide view angle requirement for consumer TV applications. This innovation is currently not available in products. As a consequence, there are no AMLCD products with both good video response and satisfactory view angles for consumer TV applications. The inventions disclosed in this patent application, appropriately adapted from tiled displays, overcome this limitation.
An excellent video response with good view angles has been demonstrated in prototype tiled, SVGA resolution, AMLCDs with a seamless appearance. The underlying designs have been described in copending patent application Ser. Nos. 09/490,776 and 09/368,921. The view angle distribution has been achieved by a combination of techniques that includes the use of highly collimated light with a sharp cut off angle together with a screen above the cover plate. The screen diffuses the highly collimated light rays outwardly, forming the desired viewing angle distribution. It therefore provides excellent brightness and hue within a view angle envelope as large as 160-170xc2x0.
One drawback to the approach used for tiled displays is that a large fraction of the light is lost in the light collimation process. The mask on the supporting back plate and the collimation mechanisms within the light box introduce the largest losses. The screen absorbs or reflects light, depending on its material components and refractive index at its interfaces. Despite these effects, improvements in light collection and recirculating technology have made it possible to predict that standard and high definition TV (SDTV and HDTV, respectively) 40xe2x80x3 display products with a brightness of 300 cd/m2 are possible at power levels of 300 W. Therefore, these same techniques may also be applied to monolithic AMLCDs. The brightness-power efficiency in monolithic displays is enhanced further because of their larger aperture ratios and less severe light collimation requirements. Finally, less light is lost in brightness and color matching in monolithic displays compared to tiled ones, from the application of software and electronics based corrections disclosed in copending patent application Ser. No. 09/396,142 for tiled displays.
Monolithic flat-panel displays made in accordance with known liquid crystal display (LCD) technologies for applications in portable computers (notebook) and desktop monitors are limited in size, due to manufacturing yields and cost. This limitation arises partly from the trend towards ever higher resolutions, rather than optimizing the designs and manufacturing of inexpensive, larger, consumer TV displays with diagonal sizes between 20 and 50xe2x80x3. Assembly tolerance requirements for tiled displays are stringent and will become more severe with the reduced size of the displays. The practical range of sizes for tiled displays at acceptable resolutions is currently estimated to be at about 30xe2x80x3 or larger. Thus the range of interest for application of direct view monolithic AMLCDs to consumer TVs spans from less than 20xe2x80x3 to about 30xe2x80x3. Monolithic display sizes much beyond 30xe2x80x3 are likely to be too expensive for mass production. At such sizes they will be in direct competition with projection displays, direct view plasma FPDs, and tiled AMLCD FPDs.
In this patent application, digital flat-panel TVs in the 24 to 40xe2x80x3 size range with resolutions from SDTV (480 lines) to HDTV (720 or 1080 lines) are chosen as illustrations for the preferred inventive design elements.
The desired lighting collimation technology for such displays is similar to that disclosed copending patent application Ser. No. 09/024,481. However, the cut-off angles for the light collimation can be substantially relaxed. In contrast, in tiled displays only about 1% of the light is allowed to escape beyond the cut-off collimation angles. Therefore, a unique light collimator design optimized for a much higher brightness-power efficiency is herein described for monolithic displays.
The majority of today""s liquid crystal display modules is digitally controlled. An optical transmission-drive voltage relationship (T-V curve) determines luminance of each sub-pixel light valve via the discretized voltage across the pixel, the LC cell in an AMLCD. Color is produced by having the light rays pass through color filter layers placed on top of the sub-pixel apertures. Three separate color filters for the red, green and blue (RGB) primary colors are the most common choice. Additive mixing of the primaries, properly weighted, produces all brightness and hue combinations in the color space. Unless otherwise noted, the T-V curve for each sub-pixel is here considered to be an effective relationship that includes the entire display system response from the electronic drive signal to the resulting luminance.
In large, tiled, liquid crystal displays, small relative placement variations of the AMLCD tiles with respect to external reference layers (for example, masks on the cover plate) result in changes in brightness and hue of the pixels due to unequal apertures. Aperture displacement may be several percent of a sub-pixel area. As a consequence, a large, tiled, flat-panel display may have an objectionable checkered appearance, due to color shifts, in spite of all efforts to geometrically hide the actual boundaries of the tile edges with external masks, located for example on the cover and back plates.
Pixels near the tile boundaries may present a different appearance, because their effective T-V curves differ from those in the interior regions. One mechanism responsible for such a difference is the variation of the LC cell gap towards the edges of the tiles. Another mechanism that often arises originates from the varying response of the liquid crystal material in the sub-pixels near the edge of the tiles. These effects may be corrected for by employing color correction algorithms and corrections to the electronic drive voltages controlling the T-V curves of appropriate sub-pixels. These color correction algorithms and techniques are disclosed in copending patent application Ser. Nos. 09/396,142, 08/649,240, and 09/173,408, and U.S. Pat. Nos. 6,020,868 and 5,668,569.
xe2x80x9cArtificialxe2x80x9d boundaries similar to actual seams in tiled displays can be created in monolithic displays by the electronics used to drive the matrix addressed pixel array. Such common optical artifacts can be observed on some notebook personal computer displays because of their xe2x80x9cdual scanxe2x80x9d electronics, in which pixels are scanned in two distinct sets. Large monolithic displays may have to be scanned in four sets (quad scan) in order to guarantee an adequate video response. Then the pixel array is divided into four quadrants. The row and column of each quadrant are driven independently using progressive scans. Such a quad scan arrangement is likely to exhibit optical artifacts at the edges of the scan regions. Other multiple scan arrangements may similarly produce optical artifacts in monolithic displays.
Another source of artifacts arises from variations among individual driver chips that cause the drive voltages at neighboring pixels to vary by as much as 10 to 20 mV. Indeed, when the voltages, timing, or other elements of such electronic drive circuits do not match precisely, artificial electronic xe2x80x9cseamsxe2x80x9d are generally created. In AMLCDs visible magnitude of the drive voltage differentials across scan region boundaries can be as small as 5 to 20 mV.
The flat-panel display structures used in this invention include monolithic and monolithic-like units in which pixels are addressed in a matrix fashion, but accessed from two edges, a single edge, or more than two edges of the array. While two-edge access is most common, other alternatives may be preferable for specific applications. Generally these different access configurations give rise to interconnect lines with a different length and cross-over count depending on the location of the pixel within the array. These differences alter the electrical characteristics of the pixels. For example the most significant effect is the kick-down effect that changes the LC cell voltage after charging from the column line via the local coupling capacitance between the cell and the row line. Both the cross-over capacitance between the row and column lines and the gate-to-drain capacitance of the TFT contribute to this coupling. Depending on the design this kick-down voltage may be as large as 2V, a significant fraction of the column voltage range. It is possible to correct for the kick-down voltage by adjusting the pixel drive signals, if this effect is uniform over the array. If the kick-down voltage varies as a function of pixel location because of array accessing, for example, these corrections become much more difficult. Techniques that work for such conditions are described hereinbelow.
The current invention provides a layout of the pixel array and its access circuits to modify the electrical characteristics in order to minimize undesirable optical, electro-optical, and ambient light aberrations and any electronic anomalies creating visually perceptible discontinuities or boundaries. These artifacts are reduced to levels that allow color correction means to be applied, as shown in U.S. Pat. No. 09,396,142. The resulting display presents luminance and chromaticity outputs from areas of originally varying optical response that become uniform within human visual tolerances.
It is an object of this invention to provide compensating means for correcting for the effects induced by any other optical, electro-optical, mechanical, or structure related anomalies in large monolithic or monolithic-like LC displays. These include, for example, the variations in the cell gap, determined by the size and placement distribution of spacer balls or fiber and by stresses generated within the assembly. Chromaticity and luminance variations at the boundaries are either corrected or smoothed over a predetermined width, so that residual variations are suppressed and boundaries or seams become visually imperceptible.
It is another object of this invention to correct for optical aberrations caused by artificial boundaries (seams) due to partition in the electronic circuits, including those for generating or transmitting pixel scans, light valve controls, and pixel drive signals, whether occurring on monolithic, monolithic-like, or tiled displays.
Another object of this invention is to electronically correct the brightness of all pixels across the interior of the pixel array, whether tiled or monolithic, so that the display presents a visually uniform luminance and chromaticity to the viewer. Such corrections are made for each display assembly and are unique to that display.
In accordance with the present invention there are provided a number of techniques for assembling large, robust monolithic and monolithic-like flat panel displays. Many techniques originally developed for creating tiled, flat-panel displays having visually imperceptible seams may be advantageously applied to these monolithic structures. By doing so, the visual sharpness, contrast, and display form factor may be improved. In addition, color and luminance balance across these monolithic displays may also be improved. Segmented row and column lines facilitate the construction of large displays beyond the thin-film RC-limit.